Methods and devices for monitoring an exhaust gas turbocharger of an internal combustion engine, in which a variable that is characteristic for the rotational speed of the exhaust gas turbocharger is ascertained in at least one operating state of an internal combustion engine, are already known.
Thus, for example, DE 10 2004 047 116 A1 discusses a rotational speed measurement using a so-called eddy current sensor. The change in the inductance of a coil having alternating current flowing through it due to the relative movement of the conductive blades of the compressor wheel of the turbocharger in relation to the coil is analyzed.
Furthermore, producing a constant magnetic field using a permanent magnet of the compressor wheel and indirectly analyzing the resulting eddy currents in the compressor blades is known. Indirectly this means that the changing eddy current in the compressor blades produces an alternating magnetic field. DE 196 23 236 A1 discusses the use of a coil as a sensor, in which an AC voltage is induced by the alternating magnetic field, whose frequency is equal to the product of compressor rotational speed and number of compressor blades.
US 2005/0017709 A1 discusses the use of a magnetoresistive sensor element for detecting the described alternating magnetic field.
The signal characteristic for the rotational speed of the compressor and thus of the exhaust gas turbocharger, which is ascertained according to the described measuring principles, for example, is generally used in an engine control unit for regulating and monitoring the turbocharger. The sensitivity of an exhaust gas turbocharger to disturbances of the oil supply is problematic. The oil supply ensures sufficient lubrication of the bearings which support the shaft between the turbine and the compressor of the exhaust gas turbocharger, and is provided from the oil circulation of the engine. Contaminants of the oil and/or the lines result in a reduced oil supply of the exhaust gas turbocharger, as a result of which the bearings may be damaged and the exhaust gas turbocharger may be destroyed.